Stereoelectronic control of photophysics:red and yellow axial and equatorial anomers of a rhenium-quinoline complex

A novel quinoline-substituted pyrimidine ligand forms two different coloured complexes upon reaction with Re(CO)(5)Br. These compounds display distinct photophysical properties that are dictated by their stereochemistry

The word as a unit of meaning. The role of context in words meaning

A unit of meaning is a word plus all those words within its contextual context that are needed to disambiguate this word to make it monosemous. A lot of research were made to study the influence of the context. They testify that there is usually in each word a hard core of relatively stable meaning and can be modified by the context within certain limits

Copolymerisation of β-butyrolactone and γ-butyrolactone using yttrium amine bis(phenolate) catalysts

The synthesis of poly(3-hydroxbutyrate-co-4-hydroxybutyrate) is reported with a family of yttrium amine bis(phenolate) catalysts via ring-opening polymerisation of β-butyrolactone and γ-butyrolactone. Poly(3-hydroxybutyrate), poly(4-hydroxybutyrate) and poly(3-hydroxybutyrate-co-4-hydroxybutyrate) have been prepared at -40 °C with a 10 M monomer concentration using a range of amine bis(phenolate) catalysts. It is found poly(3-hydroxybutyrate) synthesis is inferior under these conditions to that attainable at room temperature. In contrast, poly(4-hydroxybutyrate) synthesis achieved up to 33% conversion under these conditions. Poly(3-hydroxybutyrate) polymers containing up to 63% 4-hydroxybutyrate inclusion were obtained when β-butyrolactone and γ-butyrolactone are copolymerised, with γ-butyrolactone in excess in the monomer feed. The carbonyl resonances between 169-174 ppm in the 13C NMR spectra of this copolymer are assigned. Gel permeation chromatography on copolymers showed number average molecular weights are consistently greater than the calculated values, and the dispersities are generally greater than 1.4, demonstrating limited control by the catalysts. Despite this restricted control, these catalysts were able to convert appreciable amounts of monomers into polymers either individually or within a copolymerisation

Zirconium catalyzed alkyne dimerization for selective Z-enyne synthesis

The regioselective head-to-head dimerization of alkynes is catalyzed by a dibenzyl tethered bis(ureate) zirconium precatalyst with aniline as an additive. This system also gives outstanding stereoselectivity to furnish Z-enynes in high yields. A dinuclear reactive intermediate has been characterized, which provides a potential mechanistic rationale for the unexpected regio- and stereoselectivity in this catalytic system

Biocompatible initiators for lactide polymerization

The review summarizes recent developments in the preparation and use of new initiators for the ring opening polymerization of lactide. The review compares different classes of initiator including metal complexes, classed according to their group in the periodic table, and carbon-based initiators/organocatalysts. Emphasis is placed on the polymerization kinetics and the control exhibited by the different types of initiators. Where useful properties, such as high rates or stereocontrol, have been observed a more detailed examination of the initiator is provided. A further focus of the review is initiators displaying low toxicity and biocompatibility

Mechanistic Elucidation of Intramolecular Aminoalkene Hydroamination Catalyzed by a Tethered Bis(ureate) Complex: Evidence for Proton-Assisted C-N Bond Formation at Zirconium

A broad mechanistic investigation regarding hydroamination reactions catalyzed by a tethered bis(ureate) zirconium species, [ureate(2-)]Zr(NMe(2))(2)(HNMe(2)), is described. The cyclization of both primary and secondary aminoalkene substrates gives similar kinetic profiles, with zero-order dependence on substrate concentration up to similar to-60-75% conversion, followed by first-order dependence for the remainder of the reaction. The addition of 2-methylpiperidine changes the observed substrate dependence to first order throughout the reaction, but does not act as a competitive inhibitor. The reactions are first order in precatalyst up to loadings of similar to 0.15 M, indicating that a well-defined, mononuclear catalytic species is operative. Several model complexes have been structurally characterized, including dimeric imido and amido species, and evaluated for catalytic performance. These results indicate that imido species need not be invoked as catalytically relevant intermediates, and that the mono(amido) complex [ureate(2-)]Zr(NMe(2))(Cl)(HNMe(2)) is much less active than its bis(amido) counterpart. Structural evidence suggests that this is due to differences in coordination geometry between the mono- and bis(amido) complexes, and that an equatorial amido ligand is required for efficient catalytic turnover. On the basis of the determination of kinetic isotope effects and stoichiometric reactivity, the catalytic turnover-limiting step is proposed to be a concerted C-H, C-N bond-forming process with a highly ordered, unimolecular transition state (Delta S(double dagger) = -21 +/- 1 eu). In addition to this key bond-forming step, the catalytic cycle involves an on-cycle pre-equilibrium between six- and seven-coordinate intermediates, leading to the observed switch from zero- to first-order kinetics

Bis(phosphinic)diamido Yttrium Amide, Alkoxide, and Aryloxide Complexes: An Evaluation of Lactide Ring-Opening Polymerization Initiator Efficiency

The synthesis and characterization of a series of bis(phosphinic)diamido yttrium alkoxide, amide, and aryloxide initiators are reported. The new complexes are characterized using multinuclear nuclear magnetic resonance (NMR) spectroscopy, elemental analysis, and, in some cases, X-ray crystallography. The alkoxide complexes are all dimeric in both the solid state and in solution, as are the amide complexes substituted with iso-propyl or phenyl groups on the phosphorus atoms. On the other hand, increasing the steric hindrance of the phosphorus substituents (tert-butyl), enables isolation of mononuclear yttrium amide complexes with either 2,2-dimethylpropylene or ethylene diamido ligand backbones. The complex of 2,6-di-tert-butyl-4-methylphenoxide is also mononuclear. All the new complexes are efficient initiators for rac-lactide ring-opening polymerization. The polymerization kinetics are compared and pseudo first order rate constants, k(obs), determined. The polymerization control is also discussed, by monitoring the number-averaged molecular weight, M(n), and polydispersity index, PDI, obtained using gel permeation chromatography (GPC). The alkoxide complexes are the most efficient initiators, showing very high rates and good polymerization control, behavior consistent with rapid rates of initiation. The phenoxide and amide complexes are less efficient as manifest by nonlinear regions in the kinetic plots, lower values for k(obs), and reduced polymerization control. One of the mononuclear yttrium amide complexes shows heteroselectivity in the polymerization of rac-lactide; however, this effect is reduced on changing the initiating group to phenoxide or on changing the ancillary ligand diamido backbone group

Stereocontrolled lactide polymerisation determined by the nuclearity of the yttrium initiator

Two very rapid yttrium initiators for lactide polymerisation are reported; the nuclearity (monomer vs. dimer) of the initiator controls the stereochemistry of the polylactide produced

A series of bis(phosphinic)diamido yttrium complexes as initiators for lactide polymerization

A series of new bis(phosphinic)diamido yttrium complexes have been synthesized and fully characterized. The complexes adopt dimeric structures, both in solution and in the solid state, where one phosphinic group bonds to one yttrium center and the other bonds to two yttrium centers. The complexes have all been tested as initiators for the ring-opening polymerization of lactide; they are all highly active. The rate of polymerization is controlled by the diamine backbone substituent with the rate depending on the backbone flexibility. The order of decreasing rates were 2,2-dimethyl-1,3-propylene > trans-1,2-cyclohexylene > 1,2-ethylene >> 1,2-phenylene. The polymerization kinetics showed, in most cases, an initiation period, during which the percentage conversion and the rate of polymerization were much lower than during propagation. This was attributed to relatively slow initiation by the bulky amido group. The initiator structure was probed using 31(P){H-1} NMR spectroscopy, which showed that the dimeric structure was maintained throughout the polymerization. The initiators give rise to controlled ring-opening polymerization as shown by the linear relationship between the percentage conversion and the number-average molecular weight

Diamido-Ether Actinide Complexes as Catalysts for the Intramolecular Hydroamination of Aminoalkenes

The synthesis and characterization of a series of new diamido-thorium(IV) and diamido-uranium(IV) halide and alkyl complexes supported by three different diamido-ether ligands are reported. Reaction of ThCl4 center dot 2DME with [(RNSiMe2)(2)O]Li-2 ([(NON)-N-R]Li-2) in DME when R = Bu-t gives [(NON)-N-tBu]- ThCl5Li3 center dot DME (1), when R = (Pr2Ph)-Pr-i in diethyl ether [(NON)-N-iPr2Ph]-ThCl3Li center dot DME (3) is prepared. Reaction of UCl4 with [(NON)-N-iPr2Ph]-Li-2 in diethyl ether gives {[(2NON)-N-iPr-N-Ph]UCl2}(2) (4). Reaction of ThCl4 center dot 2DME with Li-2[((Pr2PhNCH2CH2)-Pr-i)(2)O] ([(NCOCN)-N-iPr2Ph]-Li-2) in DME gives [(NCOCN)-N-iPr2Ph]ThCl2 center dot DME (5). The addition of 2 equiv of LiCH2SiMe3 to 1 and 5 resulted in salt- and base-free [(NON)-N-tBu]Th(CH2SiMe3)(2) (7) and [(NCOCN)-N-iPr2Ph]Th(CH2SiMe3)(2) (9), respectively. Complexes 1, 3, 4, 7, and 9, as well as previously reported {(NON)-N-tBu]UCl2}(2) (2), [(NON)-N-tBu]U(CH2SiMe3)(2) (6), [(NCOCN)-N-iPr2Ph]U(CH2SiMe3)(2) (8) were examined as catalysts for the intramolecular hydroamination of a series of aminoalkenes. Complexes 6-9 were shown to facilitate the formation of 2-methyl-4,4-diphenylpyrrolidine from 2,2-diphenyl-1-amino-4-pentene at room temperature. For 9, this reaction occurs in less than 15 min, while for other diallcyls 6-8, the reaction takes less than 2 h. Dihalides 1 and 2 facilitated the same reaction at 60 degrees C in 4 h, while 3 and 4 showed no activity under the same conditions. Dialkyl complexes 7-9 were examined for further reactivity with different substrates. The uranium dialkyl 8 was more active than 7 and 9 for the cyclization of 2,2-diphenyl-1-amino-5-hexene and 2,2-diphenyl-1-amino-6-heptene, as well as more active in the cyclization of N-methyl-2,2-diphenyl-1-amino-4-pentene, a secondary amine. All three dialkyls became less active when the steric bulk of the gem-substituents was decreased from diphenyl to cyclopentyl; reactivity further decreased when the steric bulk of the substituents was decreased further to hydrogen